1. Field of the Invention
The present invention relates to a power supply apparatus for driving a capacitive load, and more particularly, to a power supply apparatus capable of generating a high voltage without transformers. This power supply apparatus can be suitably used to supply a bias voltage of a high voltage level to a charger for charging a photoconductor member or a developing unit for developing a toner image on a photoconductor member. The present invention is also concerned with a power supply apparatus suitable for a driving unit of an inkjet recording apparatus, particularly, of a type equipped with piezoelectric elements supplied with drive signals for ejecting liquid link.
2. Description of the Related Art
Conventionally, the following is known as an electrophotographic image forming apparatus to which the power supply apparatus of the above-mentioned type is applied (hereinafter referred to as first conventional art for convenience"" sake).
A conventional electrophotographic image forming apparatus is shown in FIG. 1. The surface of a photoconductor drum 100 is evenly charged at a given voltage by a primary charger 101. Then, an image is formed on the surface of the photoconductor drum 100 by exposure, so that an electrostatic latent image corresponding to the exposed image can be formed thereon. The electrostatic latent image formed on the photoconductor drum is developed by a developing unit 102, this resulting in a toner image. The toner image formed on the photoconductor drum 100 is transferred onto a transfer sheet by charging of an image transfer charger 106. The transfer sheet on which the toner image has been formed is separated from the photoconductor drum 100 by charging of a separator charger 108. Then, the image forming process ends with the step of fixing the toner image on the transfer sheet by a fixing unit (not shown).
For example, a color image forming apparatus equipped with four developing units used to sequentially form toner images of four colors on the photoconductor drum while the drum makes four turns is required to develop the toner image of color of interest without disturbing the previously developed toner image(s) of color(s). From this viewpoint, a high-voltage power supply apparatus is used which supplies, during development, one of the four developing units with a DC development bias voltage with an AC voltage necessary for enabling excellent development being superimposed thereon, while supplying the three remaining developing units with a DC voltage that prevents toner from being deposited on the photoconductor drum.
This type of high-voltage power supply apparatus is disclosed in, for example, Japanese Unexamined Patent Publication No. 8-65893, and is now illustrated in FIG. 15. The high-voltage power supply apparatus is equipped with four high-voltage power supply parts 110-113 respectively associated with four developing units 102-105. Each of the parts 110-113 has an identical configuration, and is made up of an AC voltage generator 114 that generates an AC voltage, and a DC voltage generator 115 that generates a DC voltage. The AC voltage generator 114 has a stepup transformer T for AC, having a primary winding to which an AC switching controller 116 is connected. The controller 116 turns on and off the voltage applied across the primary winding of the transformer T, so that a high AC voltage can develop across the secondary winding of the transformer T. The high voltage power supply part 111 is equipped with a voltage monitor 117 and an over current monitor 118 in order to achieve a constant-voltage output and over-current protection. The monitors 117 and 118 monitor the output voltage and the output current respectively for on/off control of the voltage applied to the primary winding of the transformer T by means of the AC switching controller 116. Thereby, the output voltage is maintained at the fixed voltage and over current is prevented from flowing in the circuit.
The DC voltage generator 115 includes a DC switching controller 119 coupled with the primary winding of another transformer T for DC. The controller 119 turns on/off a voltage applied to the transformer T, so that a high voltage can be developed across the secondary winding thereof. This high voltage is rectified by a rectifying circuit 120 composed of, for example, diodes, the resultant high DC voltage being output via a DC output controller 121.
In the above-mentioned high-voltage power supply apparatus, the AC voltage and DC voltage respectively generated by the AC voltage generator 114 and DC voltage generator 115 of each of the high-voltage power supply parts 110-113 are superimposed and the resultant bias voltages are then applied to the developing units 102-105.
However, the above-mentioned conventional high-voltage power supply apparatus has disadvantages resulting from the following. The apparatus is equipped with the four high-voltage power supply parts 110-113 respectively associated with the developing units 102-105. The developing units 102-105 are supplied with the bias voltages at the respective timings as follows. One of the developing units 102-105 subjected to development is supplied with the DC bias voltage with the AC voltage being superimposed thereon, while the three remaining developing units are supplied with only the DC voltage. Therefore, each of the high-voltage power supply parts 110-113 must be equipped with the respective transformers, namely, the AC-use transformer and DC-use transformer. Further, the apparatus is needed to have the high-voltage power supply parts 110-113 equal in number to the developing units, which act as loads. This needs a large capacity of the power supply apparatus and increases the cost.
There are proposals directed to downsizing and cost reduction of the power supply apparatus due to miniaturization and an increased number of functions of the image forming apparatus, see, for example, Japanese Unexamined Patent Publication Nos. 8-65893, 7-287620 and 8-194551.
According to Japanese Unexamined Patent Publication No. 8-65893, as shown in FIG. 16, the input lines of the primary windings of stepup transformers T1-T4 respectively associated with developing units 4a-4d serving as loads can be turned on/off independently. This arrangement makes it possible to supply a DC bias voltage with an AC voltage being superimposed thereon to the loads 4a-4d at the different timings. Further, switching means SW1-SW4 are provided on the primary sides of the transformers T1-T4, so that the switching means SQ1-SW4 can be formed by switching elements of a relatively low breakdown voltage. In FIG. 16, a reference numeral 1 indicates a photoconductor drum 1, and 11-14 indicate high-voltage power supply units. Reference numerals 15-18 indicate ac voltage generating units, and 19-22 indicate DC voltage generating units.
According to Japanese Unexamined Patent Publication No. 7-287620, as shown in FIG. 17, two switching elements 1 and 2 connected between a high-voltage DC power source 8 and ground are alternately driven, so that a high AC voltage having a rectangular waveform can be generated at a node where the elements 1 and 2 are connected in series. Further, the circuit shown in FIG. 17 allows a high DC voltage to be superimposed on the AC voltage.
Japanese Unexamined Patent Publication No. 8-194551 proposes a power supply apparatus capable of generating a DC output voltage depending on the ambient temperature. The proposed circuit does not use any transformer but employs a charge pump and a Cockcroft-Walton circuit for boosting the DC voltage. This circuit configuration enables miniaturization of the power supply apparatus.
Although the apparatus disclosed in Japanese Unexamined Patent Publication No. 8-65893 employs a smaller number of transformers, there is a limit on miniaturization because it still uses the transformers. The circuit disclosed in Japanese Unexamined Patent Publication No. 7-287620 needs the high-voltage DC source subjected to switching, and does not satisfactorily reduce the size. Further, the switching elements that switch over between the high voltage and ground are needed to have a relatively high breakdown voltage. The circuit proposed in Japanese Unexamined Patent Publication No. 8-194551 does not need any transformer, which facilitates downsizing. However, the circuit intends to realize the DC power supply, and is not used to generate the DC voltage with the AC voltage being superimposed thereon. Further, this publication does not concretely describe the Cockcroft-Walton circuit.
Japanese Unexamined Patent Publication No. 2-55577 discloses a power supply circuit using the Cockcroft-Walton circuit. This power supply circuit does not employ any general transformer in the Cockcroft-Walton circuit. This enables further downsizing of the power supply apparatus. However, the apparatus is able to generate only a positive DC voltage with respect to the ground potential, and is not able to generate both positive and negative DC voltages of the opposite polarities. Further, the apparatus is directed to generating the pure DC voltage and is not applied to generation of a DC voltage with an AC voltage being superimposed thereon.
A description will now be given of another conventional power supply apparatus (hereinafter referred to as second conventional art for convenience"" sake) suitable for driving the inkjet recording apparatus. An image recording mechanism of the inkjet recording apparatus employs a piezoelectric element, which is supplied with a drive signal. The volume of an ink chamber full of ink is varied due to deformation of the piezoelectric element, so that ink can be ejected. The drive signal that drives the piezoelectric element has a lower voltage than the voltages used for charging and developing in the image forming apparatus of the electrophotographic type, but has a higher voltage than the voltages for driving regular electronic circuits of home electric appliances. Generally, the drive signal applied to the piezoelectric element has a rectangular waveform. However, in recent years, the drive signal has been designed to have particular waveforms rather than the rectangular waveform in order to control the size and shape of ink drops and improve the rate of iterant ejection. For instance, the drive signal may have slant rising and falling edges or may have consecutive vibrations that form one ink drop. Japanese Unexamined Patent Publication No. 11-20165 discloses a circuit capable of generating the drive signal of the above type. The circuit has a voltage/current amplifier that amplifies a low-voltage pulse signal produced from a D/A converter.
However, the voltage/current amplifier disclosed in Japanese Unexamined Patent Publication No. 11-20165 needs an expensive high voltage power source and is not good in practice. There is also another disadvantage in that the piezoelectric element is constantly supplied with high energy because it is driven to always conduct.
Japanese Unexamined Patent Publication No. 4-176661 discloses a technique of generating a high-voltage pulse from a low-voltage power source. This technique proposes a circuit including a series resonance circuit including an inductance element and a flyback voltage hold circuit equipped with a rectifying element interposed between the inductance element and the piezoelectric element. As is shown in FIG. 18 that illustrates a functional block of the above circuit, a drive circuit 200 includes a switch 220 connected to a low-voltage power source 210. An inductance element 230, a rectifying circuit 240 and a selector circuit 260 are connected in series between the switch circuit 220 and a piezoelectric element TD. When the switch circuit 220 is turned on, a low voltage generated by the low-voltage power source 210 is applied to the selector circuit 260 via the switch circuit 220, the inductance element 230 and the rectifying circuit 240. The selector circuit 260 is turned on responsive to a select control signal, and the low voltage from the low-voltage power source 210 is applied to the piezoelectric element TD. A discharge circuit 250 is connected to the output terminal of the rectifying circuit 240. A switch circuit 270 is connected to the node between the inductance 230 and the rectifying circuit 240.
When an image is formed on the recording medium, the switch circuit 220 is turned on in response to a control signal, so that a supply of the low voltage from the source 210 to the inductance element 230 can be initiated. The switch 230 receives a charge control signal and starts charge control. The discharge circuit 250 receives a discharge control signal and starts discharge control. Energy applied to the piezoelectric element TD is regulated by the charge and discharge controls. Thus, the charge voltage applied to the piezoelectric element TD is retained during a predetermined period while energy applied to the piezoelectric element TD is regulated.
However, the above technique requires the inductance element 230 to have an extremely small inductance value when the piezoelectric element TD has a large capacitive value. Therefore, the circuit 200 can drive only restricted piezoelectric elements.
In short, although the first conventional art enables downsizing of the power supply apparatus because of the absence of the transformer, it cannot generate the DC voltage with the AC voltage being superimposed thereon. Particularly, the circuit with the Cockcroft-Walton circuit that does not need any transformer cannot generate the DC voltage having two polarities and that with the AC voltage being superimposed thereon. The second conventional art has a limit on usable components dependent on the capacitive value of the load to be driven, and is therefore applicable to only limited driving.
The present invention has been made in view of the above circumstances and provides a power supply apparatus and an image forming apparatus using the same.
More specifically, the present invention provides a compact, lightweight power supply apparatus suitable for various applications and an image forming apparatus using the same.
According to an aspect of the present invention, a power supply apparatus has: a boost circuit that has rectifying elements and charge storage elements associated with the rectifying elements and outputs a boosted voltage by sequentially cumulating charges in the charge storage elements in accordance with an AC input signal; and a switch circuit that switches a polarity of boosting in cumulating the charges in the charge storage elements, the boost circuit including a circuit part that supplies charges to be sequentially cumulated in positive and negative directions in accordance with the AC input signal.
According to another aspect of the present invention, a power supply apparatus has: a boost circuit having unit circuits connected in a ladder formation, each of the unit circuits including two rectifying elements and one charge storage element; and a switch circuit interchanging connections of an AC input signal and a load to input and output terminals of the boost circuit, the boost circuit including a first charge storage circuit connected between ground and one of the unit circuits located at a first stage in a positive direction and a second charge storage circuit connected between the ground and another one of the unit circuits located at a first stage in a negative direction.
According to another aspect of the invention, a power supply apparatus has: a boost circuit having 2n rectifying elements connected forwardly from a first terminal to a second terminal (1xe2x89xa6n), each of the 2n rectifying elements having first and second ends, and having charge storage elements provided so as to connect the first end of the (2i+1)th rectifying element and the second end of the (2i+2)th rectifying element (0xe2x89xa6ixe2x89xa6nxe2x88x921) and to connect the second end of the (2i+1)th rectifying element and the second end of the (2i+3)th rectifying element (0xe2x89xa6ixe2x89xa6nxe2x88x922), the second ends of the first and (2nxe2x88x921)th rectifying elements being grounded via respective charge storage elements; and a switch circuit that interchanges connections of an AC input signal and a load to the first and second terminals of the boost circuit.
According to another aspect of the invention, a power supply apparatus has: a positive-side boost circuit including first rectifying elements, first charge storage elements, and a first circuit part that supplies charges to be sequentially cumulated in the first charge storage elements in accordance with an AC signal; a negative-side boost circuit including second rectifying elements, second charge storage elements, and a second circuit part that supplies charges to be sequentially cumulated in the second charge storage elements in accordance with the AC signal; and a switch circuit that selectively supplies a load with one of outputs of the positive-side and negative-side boost circuits in accordance with a given sequence.
According to another aspect of the invention, a power supply apparatus has: a positive-side boost circuit having 2n rectifying elements connected forwardly from a first terminal to a second terminal (1xe2x89xa6n), each of the 2n rectifying elements having first and second ends, and having charge storage elements provided so as to connect the first end of the (2i+1)th rectifying element and the second end of the (2i+2)th rectifying element (0xe2x89xa6ixe2x89xa6nxe2x88x921) and to connect the second end of the (2i+1)th rectifying element and the second end of the (2i+3)th rectifying element (0xe2x89xa6ixe2x89xa6nxe2x88x922), the second end of a first rectifying element in a positive direction being grounded via a first charge storage circuit; a negative-side boost circuit having 2n rectifying elements connected reversely from the first terminal to the second terminal (1xe2x89xa6n), each of the 2n rectifying elements of the negative-side boost circuit having third and fourth ends, and having charge storage elements provided so as to connect the third end of the (2i+1)th rectifying element and the fourth end of the (2i+2)th rectifying element (0xe2x89xa6ixe2x89xa6nxe2x88x921) and to connect the fourth end of the (2i+1)th rectifying element and the fourth end of the (2i+3)th rectifying element (0xe2x89xa6ixe2x89xa6nxe2x88x922), the fourth end of a first rectifying element in a negative direction being grounded via a second charge storage circuit; and a switch circuit that selectively supplies outputs of the positive-side and negative-side boost circuits to a load.
According to another aspect of the invention, an image forming apparatus has: an image forming part; and a power supply apparatus supplying drive power to the image forming part. The power supply may be configured by any of the above-mentioned power supply apparatus.